C.22 Fixes a mistake in the code example not making a deep copy (#1573)

CppCoreGuidelines.md

@@ -4701,7 +4701,7 @@ Users will be surprised if copy/move construction and copy/move assignment do lo
         };
         shared_ptr<Impl> p;
     public:
-        Silly(const Silly& a) : p{a.p} { *p = *a.p; }   // deep copy
+        Silly(const Silly& a) : p(make_shared<Impl>()) { *p = *a.p; }   // deep copy
         Silly& operator=(const Silly& a) { p = a.p; }   // shallow copy
         // ...
     };